Japanese Unexamined Patent Publication No. 2015-215402 discloses an optical transceiver including optical components that include an optical receptacle, a wavelength-tunable light source, and an optical modulator, a printed board on which an electrical circuit is mounted, a plurality of internal fibers that optically connect the optical components, and a housing that houses the components.
Japanese Unexamined Patent Publication No. 2014-021204 discloses an optical device including an optical board, an electrical board, an optical-fiber laying component, an MEMS mirror, a plurality of internal optical fibers, a housing of a rectangular box shape, and a lid that seals the housing. The optical-fiber-laying component includes a fiber tray and a protective member that is formed of a resin and is attached to the inside of the fiber tray. The rigidity of the resin is less than the rigidity of the fiber tray. The protective member includes a fiber passage through which internal optical fibers and external optical fibers pass. Routing the internal optical fibers and the external optical fibers through the fiber passage prevents the optical fibers from contacting with the housing.
In an optical communication system according to the related art, an intensity modulation system based on ON/OFF keying of optical intensity has been used as a modulation system. In the intensity modulation system, a higher signal rate, for example 10 Gbps or more, limits a transmission range up to several km due to an influence of distortion or other optical phenomena when light propagates in an optical fiber. Coherent optical transmission technology brings long-range transmission over 1000 km or more with improvements in transmission capacity using phase modulation and polarization multiplexing of an optical signal and an increase in sensitivity using reference light (local oscillation light). Research and development of a coherent optical transceiver that provides the coherent optical transmission technology are being actively carried out.
For example, in signal transmission at a signal rate of 100 Gbps, dual polarization quadrature phase shift keying (DP-QPSK) is used as a modulation system. In the DP-QPSK, polarization multiplexing of X polarization and Y polarization for four-value phase modulation using four phases of an optical signal enable an increase in rate and capacity four times that in the intensity modulation system. Distortion of a transmission signal due to propagation in an optical fiber can be reduced at a receiving part of an optical transceiver by a digital signal processor (DSP), and wavelength dispersion compensation using a dispersion correction fiber in the related art becomes unnecessary.